1. Field of the Invention
The present invention relates to a vibration limit switch system. More particularly, the present invention relates to vibration limit switch system that eliminates thermally and other induced shearing forces.
2. Description of the Related Art
The detection of the limiting status plays an important part in industrial processes, in particular for liquids and bulk materials. In the determination of the limiting status the attaining of a defined filling height is detected and converted into a switching command. The switching command then starts or stops, for example, a filling apparatus so that an overfilling of a container is prevented. A detection of the limiting status can be realized, for example, in liquids, pastes, powders, or coarse bulk materials.
Vibration limit switches like those known from the prior art comprise a piezoelectric transmitting- and/or receiving unit, a membrane that can be put in oscillation and a mechanical oscillation arrangement that is coupled to the membrane, whereby the piezoelectric transmitting- and/or receiving unit is adhered directly or via an adaptation layer by means of an adhesive layer to the membrane. An adhering of the piezoelectric transmitting- and/or receiving unit to the membrane has a distinctly simpler construction in comparison to a coupling via a traction bolt, by means of which the transmitting- and/or receiving unit is tensioned to the membrane, and thus makes possible a more favorable manufacture of the vibration limit switch.
FIG. 1 shows a section through a conventional vibration limit switch 1 known from the prior art. The vibration limit switch shown comprises a housing 10 that is closed on the front side by a membrane 4 which can be put in oscillation. The membrane 4 is as a rule designed to be circular and has a continuously uniform thickness, as is shown in FIG. 1. A mechanical oscillating arrangement 5 is arranged on the front side of the membrane 4 which oscillating arrangement in the present exemplary embodiment is an oscillation fork. The oscillation fork 5 is stimulated to its resonance frequency by oscillations of the membrane 4 and begins to oscillate with this frequency. When the oscillation fork 5 is covered with filling materials physical contact results and the frequency of the oscillation fork 5 necessarily drops, so that the attaining of a limiting status can be detected and converted into a switching signal.
The membrane 4 can be put in oscillation via a piezoelectric transmitting- and/or receiving unit 2 which is arranged inside the housing 10. The piezoelectric transmitting- and/or receiving unit 2, also called piezodrive in the following, is adhered in the present exemplary embodiment to the membrane 4 via an adaptation layer 7. The sole scope of the related art shows that both the adaptation layer 7 as well as the membrane 4 are constructed with level planar surfaces that face one another so that an adhesive layer 9 arranged between the membrane 4 and the adaptation layer 7 has a constant thickness over its entire extent.
The adaptation layer 7 is designed in the present exemplary embodiment as a so-called adaptation ceramic material that should ensure a graduated transition of the thermal coefficient of expansion between the piezodrive 2 and the membrane 4.
The thermal coefficient of expansion of the membrane 4, that typically consists of high-grade steel, is approximately 16×10−6 K−1, whereby the thermal coefficient of expansion of the piezoelectric transmitting- and/or receiving unit Z is typically in the range of 4×10−6 K−1. The difference in the thermal expansion of membrane 4 and piezodrive 2, that differs approximately by a factor of 4, can be adapted to one another by an adaptation layer 7 of material with a thermal coefficient of expansion of approximately 8×10−6 K−1. The piezodrive 2 is adhered, for its part, to the adaptation layer 7 so that the adaptation layer can transfer the mechanical oscillations of the piezodrive 2 onto the membrane 4. In addition, a ceramic adaptation layer 7 ensures an electrical insulation between electrodes provided on the piezodrive 2 that serve to control the piezodrive 2 and between the metallic membrane 4 as well as between the oscillation fork 5 arranged on the latter.
It is well recognized in the vibration limit switches known from the prior art and with the design described above, as a strong disadvantage that tears frequently occur in the adhesive layer 9 in spite of the adaptation layer 7. These tears in the adhesive layer 9 are traced to thermally induced shearing forces that result in a fatigue of the adhesive layer 9 in spite of an adaptation of the thermal coefficient of expansion of the materials used, so that a formation of tears and therewith a defect of the vibration limit switch 1 can not be prevented. To date, no solution to this strong concern has been presented in the related art.
Accordingly, there is a need for an improved vibration limit switch system, that overcomes the strong disadvantages of the prior art and makes available an improved vibration limit switch.